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Do you think the Dynojet needle will work well with the Dynojet 98 main?
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Yes, that should be in ballpark more than where you are now.
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Originally Posted by vtrider
Ok so been reading more about CV carbs and I guess I can see how cv slide/needle height can be a function of engine RPM without needing to consider load. More cycles per second, a certain volume of air per second exhausted from the cylinders, greater vacuum..
I’d like to see a graph of engine load, throttle position, vacuum pressure and rpm. Think that would be mighty helpful in understanding this better. See if can track something like that down from google.
Ok well Wikipedia says “The mass flow through the engine is the product of the rotation rate of the engine, the displacement of the engine, and the density of the intake stream in the intake manifold” so almost totally linear to rpm excluding blips relative to fuel mixture.
And from mechanics stack exchange the definition of calculated engine load is:
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Yup, you got it. But Wikipedia/StackExchange is simplistic at best and not very accurate in this matter (don't get me started on mechanic's old-wives tales). Look up "Helmholtz resonance" which deals with pulses going back & forth through engine due to valves opening & closing. There's two chains of pulses, one on intake and one in exhaust. At certain RPMs, they work together to use exhaust flow to pull in intake charge and give you strong vacuum. At other RPMs, exhaust actually backs up out into carbs and fights incoming air. This lowers vacuum and messes with your carb's fuel-metering.
There's also slight time-delay in how slides responds to changes in vacuum. So amount of fuel being sent into engine may not match precisely amount of air going in at that time. There's actually variable-range where stumble occurs as function of load X RPM.
Here's 3D map for you of fuel-injected volumes above/below VE=1 (what Wikipedia assumes and StackExchange’s formulae) to achieve 14.7:1 stoich mixtures. Table is LOAD X RPMs, which is proxy for air-mass and is far from linear. And... all these different maps are for
exact same engine!. Well, different ones of exact same model with different rates of wear and states of upgrades & tune.
Maps were created on eddy-current load-dyno which holds engine at exact 10% throttle X 500rpm increments for 1-second while AFR is sampled. Then load & RPM incremented to next data-point and AFR sampled and so on.
Idle is lowest corner nearest you, RPMs increase to right and airflow (load) increases to left. You see where VE is highest in mid-RPMs (torque peak), extra fuel added is highest. Then as RPMs increase past torque-peak, VE decreases, torque decreases and additional fuel added is reduced. If an engine had VE=1 all across board as Wikipedia assumes, these maps would be perfectly flat.
For better understanding of how engines work beyond 5% intro Wikipedia gave, you'll want to read these:
Engineering Fundamentals of Internal Combustion Engine - Pulkrabek 2003 Prentice Hall
Flow and Combustion in Reciprocating Engines - Springer, Arcoumanis, Kaminoto 2009 Verlag Berlin Heideberg
Performance Tuning Theory & Practice - Four Strokes - Bell 1981 Haynes
Tune & Modify Automotive Engine Management Systems - Hartman 2004 MBI Publishing
I'm also not certain that issue isn't related to carb-cleaning and how thoroughly you cleaned carbs. Were O-rings and seals replaced? Hidden secret fuel-passages in carb-body scrubbed and flossed? Micro soda-blasting is really needed due to scrubbing shoving fragments of dried petrol deeper into carb-body.